This invention relates generally to transmission of data by modulation of the power factor of a power transmission line, and more particularly to an improved receiver for such data.
U.S. Pat. No. 3,959,767 to Smither et al describes a data transmission system for transmitting information from a well bore to the surface over the line carrying power down the well bore. By switching a reactance into the power line in the well bore, the phase angle between the voltage and current is modified. This power factor modulation is used to transmit, in a digital form, information, which is then detected and decoded at the surface.
In constructing and testing the invention of Smithers et al, a receiver was first used which measured the amplitude of the current at the times the voltage was zero (the zero crossings of the voltage). Errors were found to occur in the detected data which appear to result because of changes in the power factor as the length of the coaxial power line changed, and also because of noise which appeared to affect the zero crossings.